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The invention relates to a heat sealable and peelable film. The invention also relates to methods of making and using the heat sealable, peelable film.
Heat sealable and peelable films (also referred to herein as xe2x80x9cpeelable sealsxe2x80x9d) are ubiquitously employed on a large scale for temporarily closing containers that include, for example, food products or medical devices. During use, a consumer tears away the peelable film. To gain consumer acceptance, a number of characteristics associated with a heat sealable and peelable film are desired. For example, the film should provide a leak-proof closure of the container or bag. To seal a bag, heat sealing is commonly used. Various apparatus have been constructed for the purpose of forming bags while simultaneously filling the bags with the desired contents. These apparatus are commonly known as vertical form-fill-and-seal and horizontal form-fill-and-seal machines.
These machines typically have forming collars or bars that shape a flat piece of film into a tubular shape of a bag. Hot metal sealing jaws are moved from an open position to a closed position, contacting the film in order to seal it into a bag shape. During the sealing process, the outer layer of the film comes into direct contact with the hot metal surface of the sealing jaws. Heat is thus transferred through the outer layer of the film to melt and fuse the inner sealant layer to form a seal. Generally, the outer layer has a higher melting temperature than the inner sealant layer. As such, while the inner sealant layer is melted to form a seal, the outer layer of the film does not melt and is not stuck to the sealing jaws. After the sealing jaws reopen, the film is cooled to room temperature.
Before the inner sealant layer is cooled to room temperature, it should be able to maintain its seal integrity. The ability of an adhesive or sealant layer to resist creep of the seal while it is still in a warm or molten state is generally referred to as xe2x80x9chot tack.xe2x80x9d To form a good seal, the hot tack of the sealable and peelable film should be adequate.
Besides adequate hot tack, it is also desirable to have a low heat seal initiation temperature which helps to ensure fast packaging line speeds and a broad sealing window which could accommodate variability in process conditions, such as pressure and temperature. A broad sealing window also enables high speed packaging of heat sensitive products, as well as, provides a degree of forgiveness for changes in packaging or filling speeds.
In addition to the xe2x80x9csealablexe2x80x9d characteristic of a sealable and peelable film, it should also have a desired xe2x80x9cpeelablexe2x80x9d characteristic needed to provide an easily openable seal on a package or bag. Peelability generally refers to the ability to separate two materials or substrates in the course of opening a package without compromising the integrity of either of the two. The force required to pull a seal apart is called xe2x80x9cseal strengthxe2x80x9d or xe2x80x9cheat seal strengthxe2x80x9d which can be measured in accordance with ASTM F88-94. The desired seal strength varies according to specific end user applications. For flexible packaging applications, such as cereal liners, snack food packages, cracker tubes and cake mix liners, the seal strength desired is generally in the range of about 1-9 pounds per inch. For example, for easy-open cereal box liners, a seal strength in the range of about 2-3 pounds per inch is commonly specified, although specific targets vary according to individual manufactures requirements. In addition to flexible packaging application, a sealable and peelable film can also be used in rigid package applications, such as lidding for convenience items (e.g., snack food such as puddings) and medical devices. Typical medical packages have a seal strength of about 1-3 pounds per inch.
Additional desired characteristics for a heat sealable and peelable film include a low coefficient of friction and good abuse resistance. A low coefficient of friction ensures that the sealant layer can be processed smoothly and efficiently on fabrication and packaging equipment and is particularly important for vertical form-fill-and-seal packaging. Good abuse resistance and toughness is desired, for example, in cereal box liners to withstand tears and punctures from irregularly-shaped, rigid cereals. Additional characteristics include taste and odor performance and barrier or transmission properties.
Heat sealable and peelable films are generally made from one or more polymeric resins. The resulting characteristics of a heat sealable and peelable film depend largely upon the type of the resins used to form the film. For example, ethylene vinyl acetate (EVA) and ethylene methyl acrylate (EMA) copolymers provide excellent heat sealing properties. However, the seals produced with these copolymers are such that separation usually cannot be achieved without damage to the film. To alleviate this problem, polybutylene is mixed with an EVA polymer to produce a heat sealable and peelable film. Although the peelability of the film is improved, the heat sealable and peelable film has some unpleasant odor due to the presence of EVA. In addition to using polybutylene, some ionomers, such as SURLYN(copyright), is mixed with EVA to produce a heat sealable and peelable film. While the film is peelable, it causes stringiness or xe2x80x9cangel hairxe2x80x9d upon separation of the film. Moreover, ionomers are generally expensive and may have some odor as well.
Although a number of resins systems have been employed to make a heat sealable and peelable film, there continues to exist a need for an improved heat sealable and peelable film with consistent seal strength. It is desirable that the resin system used to produce the heat sealable and peelable film has a relatively lower seal initiation temperature and a relatively broad heat sealing window. It is also desirable that the heat sealable and peelable film is relatively age-resistant and has a relatively lower coefficient of friction and good abuse resistance and toughness.
The aforementioned needs are fulfilled by embodiments of the invention in one or more of the following aspects. In one aspect, the invention relates to a peelable seal which comprises a mixture of at least two immiscible polymers; the first polymer forms a continuous phase and has a shear viscosity xcex71 at a temperature of about 230xc2x0 C. and a shear rate of about 100 radian/second; and the second polymer is dispersed in the continuous phase and has a shear viscosity xcex72 at a temperature of about 230xc2x0 C. and a shear rate of about 100 radian/second. Moreover, the two polymers define a shear viscosity differential: xcex94=|(xcex71xe2x88x92xcex72)/xcex71|, and the shear viscosity differential xcex94 is less than about 100%. The peelable seal may be a monolayer or a multilayer. In a multi-layered peelable seal, it may include at least a base layer and a skin layer which is formed from the mixture of the at least two immiscible polymers. The peelable seal may also include two base layers and a skin layer which is formed from the mixture of the at least two immiscible polymers. In some embodiments, the shear viscosity differential xcex94 may be less than about 50%, less than about 30%, less than about 20%, less than about 10%, less than about 5%. The shear viscosity differential xcex94 may also be zero or substantially close to zero.
In some embodiments, the first polymer is an ethylene polymer. The ethylene polymer may have a melt index in the range from about 0.1 to about 20, preferably from 0.6 to about 10, and more preferably from about 1.5 to 3. In still other embodiments, the second polymer may be a propylene polymer. The propylene polymer may have a melt flow rate from about 0.01 to about 2, preferably from 0.1 to about 1, and more preferably from about 0.3 to about 0.6. In other embodiments, the first polymer is an ethylene polymer with a melt index in the range from about 1.5 to about 3, and the second polymer is a propylene polymer with a melt flow rate from about 0.3 to about 0.6.
In another aspect, the invention relates to a peelable seal which comprises a mixture of an ethylene polymer with a melt index in the range from about 0.1 to about 20 and a propylene polymer with a melt flow rate in the range from about 0.01 to about 2. Moreover, the ethylene polymer forms a continuous phase of the peelable seal, and the propylene polymer is dispersed in the continuous phase of the peelable seal. The peelable seal may be a monolayer or a multilayer. In a multi-layered peelable seal, it may include at least a base layer and a skin layer formed from the mixture of the ethylene and the propylene polymers. In some embodiments, the peelable seal may include two base layers and a skin layer formed from the mixture of the ethylene polymer and the propylene polymer. The propylene polymer is uniformly dispersed in the continuous phase of the ethylene polymer. The ethylene polymer may have a melt index in the range from about 0.6 to about 10, preferably from about 1.5 to about 3. The ethylene polymer may have a density from about 0.86 g/cc to about 0.97 g/cc, preferably from about 0.86 g/cc to about 92 g/cc, and more preferably from about 0.88 g/cc to about 0.92 g/cc. The ethylene polymer may be selected from high density polyethylene, low density polyethylene, linear low density polyethylene, very low density polyethylene, and ultra low density polyethylene. The ethylene polymer may be produced by a single site catalyst, a metallocene catalyst, a constrained geometry catalyst. The ethylene polymer may be a substantially linear ethylene polymer or an ethylene polymer with long chain branching. The propylene polymer may be a homopolymer, a copolymer, or an interpolymer. The propylene polymer may have a melt flow rate in the range from about 0.01 to about 2, preferably from about 0.1 to about 1, and more preferably from about 0.3 to about 0.5. In some embodiments, the ethylene polymer has a melt index in the range from about 1.5 to about 3, and the propylene polymer has a melt flow rate of less than about 0.6. In other embodiments, the ethylene polymer and the propylene polymer define a shear viscosity differential: xcex94=|(xcex7exe2x88x92xcex7p)/xcex7e|, in which xcex7e and xcex7p are the respective shear viscosity of the ethylene polymer and the propylene polymer at a shear rate of about 100 radian/second and a temperature of about 230xc2x0 C. Moreover, the shear viscosity differential xcex94 is than about 100%. Preferably, the shear viscosity differential xcex94 is less than 50%, less than 25%, less than 10%, less than 5%, or zero or substantially zero.
In yet another aspect, the invention relates to a method of making a peelable seal. As; The method includes: (a) obtaining a ethylene polymer with a melt index in the range from about 0.1 to about 20; (b) obtaining a propylene polymer with a melt flow rate in the range from about 0.01 to about 2; (c) mixing the ethylene polymer and the propylene polymer to obtain a blend therefrom, and (d) forming a peelable seal from the blend. Moreover, the ethylene polymer forms a continuous phase in the peelable seal, and the propylene polymer forms a dispersed phase in the peelable seal.
In still another aspect, the invention relates to a method of making a peelable seal. The method includes: (a) obtaining a first polymer with a shear viscosity xcex71 at a temperature of about 230xc2x0 C. and a shear rate of about 100 radian/second; (b) obtaining a second polymer with a shear viscosity xcex72 at a temperature of about 230xc2x0 C. and a shear rate of about 100 radian/second; (c) mixing the first polymer and the second polymer to form a blend; and (d) forming a peelable seal from the blend. Moreover, the first and second polymers define a shear viscosity differential xcex94=|(xcex71xe2x88x92xcex72)/xcex71| less than 100%. The first polymer forms a continuous phase in the peelable seal and the second polymer forms a dispersed phase in the peelable seal.
In yet still another aspect, the invention relates to a polymer blend composition for a peelable seal. The polymer blend composition comprises a blend of an ethylene polymer with a melt index in the range from about 0.1 to about 20 and a propylene polymer with a melt flow rate in the range from about 0.01 to about 2. The ethylene polymer is capable of forming a continuous phase of the peelable seal, and the propylene polymer is capable of being dispersed in the continuous phase of the peelable seal.
In one aspect, the invention relates to a polymer blend composition for a peelable seal. The polymer blend composition comprises a blend of at least two immiscible polymers; a first polymer capable of forming a continuous phase and a second polymer capable of being dispersed in the continuous phase. The first polymer and the second polymer have a shear viscosity xcex71 and xcex72 at a temperature of about 230xc2x0 C. and a shear rate of 100 radian/second. Moreover, the two polymer define a shear viscosity differential: xcex94=|(xcex71xe2x88x92xcex72)/xcex71|, and the shear viscosity differential is less than about 100%.
Additional aspects of the invention and characteristics and advantages provided by embodiments of the invention are apparent with the following description.